This invention relates to the cubic form of boron nitride and its formation or transformation from the hexagonal form of boron nitride. More particularly, this invention relates to the production of polycrystalline CBN abrasive particles of a controlled particle size from HBN powders.
Three crystalline forms of boron nitride are known: hexagonal boron nitride (HBN), a soft graphitic form similar in structure to graphite carbon; wurtzitic boron nitride (WBN), a hard hexagonal form similar to hexagonal diamond; and cubic boron nitride (CBN), a hard zinc blend form similar to cubic diamond. The three boron nitride crystal structures may be visualized as formed by the stacking of a series of sheets or layers of atoms. FIGS. 1-a through 1-c of U.S. Pat. No. 4,188,494 illustrate these structures in greater detail. In HBN crystals, the boron and nitride atoms bonded together are in the same plane as stacked layers. In the more dense CBN crystal structures, the atoms of the stacked layers are puckered out of plane. In addition, bonding between the atoms within the layers of an HBN crystal is predominantly of the strong covalent type, with only weak van der Waals bonding between layers. In CBN crystals, strong, predominantly covalent tetrahedral bonds are formed between each atom and its four neighbors.
Methods for converting HBN into CBN monocrystalline and polycrystalline particles are well known. U.S. Pat. No. 2,947,617 describes a method for preparing cubic boron nitride by subjecting a hexagonal form of boron nitride, in the presence of specific additive materials or catalysts, to very high pressures and temperatures within the cubic boron nitride stable region defined therein by the phase diagram of boron nitride in FIG. 1. Methods for the conversion of HBN to CBN in the absence of catalyst using higher pressures and temperatures are described in U.S. Pat. No. 3,212,852. See also Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride (VI)," and Ichimose et al., "Synthesis of Polycrystalline Cubic Boron Nitride (V) ," both in Proceedings of the Fourth International Conference of High Pressure, Kyoto, Japan, 1974, pp. 436-445; U.S. Pat. No. 4,016,244; JP SHO 49-27518 (Wakatsuki et al.); JP SHO 49-30357 (Wakatsuki et al.); JP SHO 49-22925 (Wakatsuki et al.); U.S. Pat. No. 3,852,078 (Wakatsuki et al.); Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride (V)," Mat. Resp. Bul. 7, 999-1004 (1972); and GB 1,317,716 (Serota).
It is the cubic form of boron nitride which finds use as an abrasive material typically in the form of a cluster compact, a composite compact, or as particles agglomerated together to form an abrasive tool such as a grinding wheel. In cluster compacts and composite compacts, the abrasive crystals are chemically bonded together, typically in a self-bonded relationship. In contrast, the particles of an abrasive tool are physically bonded together with the aid of a matrix. U.S. Pat. Nos. 3,136,615 and 3,233,988 provide a detailed description of certain types of cluster compacts and methods for their manufacture. U.S. Pat. Nos. 3,743,489 and 3,767,371 provide a description of composite compacts and their manufacture.
Conventional methods for obtaining CBN particles for abrasive tools involve the conversion of fine-sized HBN powders to a CBN mass using the high pressure, high temperature methods described above. After formation, the large CBN mass is extensively milled to obtain particles which are of varying size. The particles are then sieved to obtain particles of a particular mesh size. Using this procedure results in the generation of a significant amount of particles too small for use, referred to as non-valued fines. Excessive milling of the CBN mass requires significant energy and also tends to round the particle edges, which reduces their performance in grinding applications.
It is desirable to provide a more efficient, cost-effective method for producing CBN particulates with reduced milling. U.S. Pat. No. 5,015,265 (Corrigan and Slutz) describes coating HBN particles with non-sinterable coatings prior to conversion of the HBN particles to CBN. This provides coated CBN particles of a desired size with reduced milling. It is desirable to reduce the milling necessary in providing uncoated CBN particles of a controlled size.